-looks weird but sub lets it avoid leech seed, paralysis, and burn
-serves a secondary shuffling role with dragon tail
-can take on more offensive role with outrage (can use more attack EVs)
-chesto berry aims to have a quick sweep and can replenish lost hp from substitute and weaker attacks
-rest does not need to be used with chesto berry
-earthquake gives strong neutral coverage and ohkos heatran so you don't have to eat its flamethrowers. it also prevents you from having to take iron barb damage every time. if dialga is last 'mon it will have trouble sweeping with even +6 dragon tail, also PP issues.
-can set up 101 hp subs
-sets up a multitude of support 'mons, Dtail lugia, chans / bliss , ferrothorn, forretress, arceus-grass, crogre, arceus rock, arceus dark, arceus psychic, etc
-best used with spikes!!
-works well in all weathers, most notably sand as it avoids residual
-screwed by fighting and ground arceus
-opposing dialga are a problem without earthquake
-lugia with whirlwind owns this set unless a lot of speed is used
-its slow
-can't take too many strong hits

[ADDITIONAL COMMENTS]

-maximize hp for 101 hp subs, rest is poured in SDef to maximize special bulk
-a more offensive spread can be used with outrage, actually aiming to sweep
-leftovers is the main item, and can still be used with rest, however, you're completely vulnerable to phazing for 2 turns
-support w/ ferrothorn, forretress, skarm to set up spikes; deo-s is a decent partner too, as this set works well on quickstall teams
-fairly self-reliant outside of hazard support
-rain support to dampen fire attacks
-164 speed to beat 0 speed lugia and phaze it out first with dragon tail. sub + pressure will quickly drain dragon tail pp though.

I think trickroom actually rejected Dragon Tail on BU Rest talk Dialga due to it being too weak 0_0, I think the same problem might come up here but Sub is a really wierd way to say FU to Ferro so idk. Also make sure to note that if you do decide to setup on Ferro / Forry that you better be prepared to have 3 layers of spikes around you. Also if you get phazed when using Dragon Tail due to Tail being simply too weak you know somethings not right -_-, I actually think Outrage should be the main slash but again I don't know...

Dragon Claw works too for pp issues and when you don't want to risk hurting yourself in confusion for that one turn that Lugia ends up phazing you out, maybe a mention in AC? You could also ignore me but w.e.

MENTION THIS IS VULNERABLE TO SPINNING. FORRY WILL LOL (no seriously, maybe spin blocker support? This Dialga DOES take advantage of its existence though so idk) although you do get the last laugh if you 6-0 with a +6 outrage :P.

I don't agree with Rest being considered as an option on this set. If you're going to use Rest, Bulk Up, and another non-attacking move on the same set, why not just use Resttalk? Don't get me wrong, Sub Dialga is great and Resttalk Dialga is great, but why try to combine the two? I'd change the slash to something else. Brick Break maybe? Another dragon attack?

What Shrang and lol have said is also true - You need to mention spinblock support and you need to address the speed problem (not speed creep, mind you, but the speed problem).

Have you considered Protect on the last slot? It could be like Sub/BU/DTail/Protect, much like the Kyurem set that terrorized UU at its conception (Sub/HC/DTail/Protect). Dialga has better typing and defense boost, so maybe it could work?

Idk I'm not very sold on this set because it doesn't seem to do enough damage for its lifespan. Maybe if you could keep it alive with Wish...idk. Some logs would be nice.

I tested this out a fair bit and I got mixed results. The thing is that Dialga can set up on half the stuff in Ubers especially on Support mons and Genesect too :D Most balanced / stall teams not having a strong physical attacker with a super effective move will be promptly set up on and crushed. However a major flaw of this et was the lack of recovery. Setting up a sub for a turn only to have it broken the next is INCREDIBLY wasteful for this Dialga. Yes, like poppy said on irc this is a niche set but it does have its uses. Leaning towards an approval.

I think Fire Blast can be an option for this set. It's more useful than Earthquake vs all the Spikes user except Tentacruel who does not enjoy boosted Dragon Tails. Earthquake nails Excadrill regardless of weather though.

Jibaku: Idk I'm not very sold on this set because it doesn't seem to do enough damage for its lifespan.

I think I agree with that. Dialga using Substitute means you lose some coverage and you are not supporting the team much. Instead of setting up Sub on Ferrothorn, you could directly roast it with Fire Blast. Also Careful max Special Defense Bulk Up Dialga on a no recovery variant? :(

Don't run more Speed, bulk or more power is way more important than outspeed min Speed Lugia. I like Outrage more, because it is a lot stronger and you can actually sweep with it. Dragon Tail faces major PP issues, because its weak and many Pressure users ( Mewtwo, Dialga, Palkia, Giratina, some Lugia ) around. Giratina-O is better for those Sub Dragon tailing as Giratina-O is immune to Spikes, bulkier and more importantly blocks spin.

Mainly because it can't viably keep many spinners from spinning, especially forry, I don't really think this set is too viable.

Admittedly there are other pokemon who sub-phaze, like lugia and gira-o, but in the case of the former, it hard walls all variants of many commonly used sweepers and threats, and in the case of the latter actually sets up while preserving the hazards it needs to do damage by virtue of its ghost typing.

Dialga has neat resistances, for sure, but it really cant switch into every variant of stuff like kyogre, and can even lose its sub to mono attacking sets.

Of the list you listed, you can't really set up on the wall arceus forms, as they have a tendency to run WoW

You obviously beat psych arceus, and you can phaze arceus dark, but you have to be at +2 to be able to break its sub, so your going to take some decent damage in the process (excuse my obvious bias).

In short, the turns you use to set up, and more importantly the health you lose, makes you a poorer check, which this set attempts to balance out by being able to shuffle a team. But in my opinion, that's canceled out by the ability of spinners to spin on you, and the fact that the loss of these turns and health makes you an even shakier check to certain threats than dialga already is, as it often barely gets away with beating common sets of arceus, kyogre, and many many other mons by the skin of its teeth and favorable hazard conditions.

Yeah, I personally don't think shuffling is all that great when spinners can easily just spin away your hazards and force you to do very little. If Dialga is to have a Sub + Bulk Up, I reckon Outrage / Dragon Claw would be a much better option as the STAB move.

Yeah I'm gonna reject this. While it was fun to playtest with that, being a hazard fodder is really a pain in the ass, and honestly, you can't really setup that easily. You try to setup on Groudon? Roar. Kyogre also breaks your Substitute with Ice Beam, and sure, you can setup Bulk Up on it, but you take too much damage. You take too long to setup, as syrim said, and as shrang said, spinners can just come and take away your hazards. No recovery makes this set extremely risky, and Rest takes some of your momentum.

Also Dialga is really missing a lot things with this set, it no longer checks Kyogre well and you lose a ton of power compare to the Support set. Dragon Tail is piss weak and Earthquake tickles Forretress. Sorry.

Groudon still breaks past you with repeated Earthquakes even if you boost as it switches in. Dialga lacks recovery and that sucks with a set like this.